I. Field of the Invention
This invention relates generally to wrapping and packaging machines and, more particularly, to a horizontal wrapping machine utilizing a microprocessor-based control system having separate SCR drives driving separate permanent magnet drive motors which are controlled independently of one another.
II. Discussion of the Prior Art
In a horizontal wrapping machine, a continuous film of packaging material is supplied from a roll and drawn past a former which shapes the film into a continuous tube of packaging material. Products to be wrapped are supplied through the former into the tube of packaging material such that the products are spaced apart from one another in the tube. The opposed edges of the film of the tube are juxtaposed and longitudinally sealed, and the tube of packaging material is then cut and transversely sealed as each product, carried within the tube, passes through a sealing and cutting station. In this way, an individual sealed wrapping is formed about each product.
The film is formed in the former such that the lateral edges of the film, when the tube is formed, extend downwardly from the center of the film tube in a side-by-side relationship. One or more pairs of finwheels, rotating about vertical axes, engage opposite sides of the downwardly extending pair of film edges to drive the film toward the cutting and sealing station. At least one pair of finwheels in the finwheel assembly may be heated, serving to heat seal the downwardly extending film edges together to seal the tube of heat sealable film as they rotate to advance the film off its supply roll. Other so-called "cold seal film" do not need heat but, instead, use the pressure of one or more finwheel assembles to create the seal.
As the now-enclosed tube of film carrying products which are spaced apart from one another advances past the sealing and cutting station, opposed rotary cut/seal heads, one containing a knife member and the other an anvil, come into engagement with the film tube between each successive pair of products. The cut/seal head may also include heated members so as to seal the film as it is cut to thereby form individual sealed packages, each containing a non-wrapped product.
One typical horizontal wrapping machine as disclosed in U.S. Pat. No. 4,574,566, and assigned to the present applicant, teaches a horizontal wrapping machine having several individually controlled servo-controlled motor drives which are controlled in response to the measured film velocity. One motor is used to drive the product in-feed conveyor, others for the finwheel assemblies and yet another for the cut/seal head. Servo-controlled motor drives each have a closed-loop servo-control circuit including a motor which is driven by a summing amplifier. The summing amplifier receives as a feedback signal the actual motor velocity and receives as a control signal a desired motor velocity. While these servo-controlled motor drives are very well suited to be implemented in horizontal wrapping machines, they tend to be quite expensive. Basically, the high cost of the servo-controlled motor drives is justified by the quick response rate and the linearity of the motor speed in response to the control signal. Thus, quick start and stop times can be realized using servo-controlled motor drives.
Since horizontal wrapping machines require a substantial financial investment by a business, it is desirable to provide a low cost horizontal wrapping machine without sacrificing any of the major features of individually controlled motor drive wrapping machines, such as easy product changeover or the accurate control of package lengths Since horizontal wrapping machines are usually continuously used, the starting and stopping times of the machine are perhaps a less important consideration to the average business. Thus, a horizontal wrapping machine which utilizes motors which are less expensive, albeit having slower response times, is desirable in the industry to provide an affordable alternative to some of the more expensive high-performance horizontal wrapping machines currently available.
Motors, such as SCR wound permanent magnet motors, typically respond in a very linear manner to control signals, but have much slower response times as compared to DC servo motors. Further, permanent magnet motors do not inherently contain feedback control means to sense a precise motor speed in response to an input signal. Thus, using permanent magnet motors would require a discrete feedback means to monitor the actual speed of the motor in response to a particular control signal.